如何在类方法的装饰器中访问该类的静态成员？

想要用TypeScript 5.0新装饰器API，让类方法自动使用同类中指定的静态属性作为参数。当前实现的装饰器无法访问目标静态属性，调用方法时newArgs为undefined。

原装饰器代码：

type TargetFunction<This, Args extends unknown[], Return> = (this: This, ...args: Args) => Return;

class Throwable extends Error {} // 仅用于演示

export interface ITestParameters {
    dataProvider?: string;
    expectedExceptions?: typeof Throwable;
}

export const Test = <T extends ITestParameters>(param: T) => {
    return <This, Args extends unknown[], Return>(target: TargetFunction<This, Args, Return>,
        context: ClassMethodDecoratorContext<This, TargetFunction<This, Args, Return>>) => {
        return function (this: This, ...args: Args): Return {
            if (param.dataProvider) {
                const newArgs = (this as any)[param.dataProvider] as Args;
                return target.call(this, ...newArgs) as Return;
            }

            return target.call(this, ...args) as Return;
        };
    };
};

测试类代码：

class SubList {
    @Test({ dataProvider: "modifiable" })
    public testMod(list: number[], from: number, to: number): void {
        console.log(list, from, to);
    }

    public static modifiable: [number[], number, number] = [[1, 2, 3], 4, 5];
}

(new SubList()).testMod([], 0, 0); // 报错：newArgs is undefined

问题根源是实例this无法直接访问类的静态成员，需要通过实例的constructor属性获取所属类，再访问静态属性。

修改装饰器中返回的包装函数：

return function (this: This, ...args: Args): Return {
    if (param.dataProvider) {
        // 通过constructor获取类，访问静态属性
        const Class = this.constructor as Record<string, unknown>;
        const newArgs = Class[param.dataProvider] as Args;
        return target.call(this, ...newArgs) as Return;
    }

    return target.call(this, ...args) as Return;
};

修改后，调用(new SubList()).testMod([], 0, 0)时，会自动使用静态属性modifiable的值作为参数，输出[1, 2, 3] 4 5。

如果需要更严格的类型检查，可以给Class添加类型约束，确保它包含指定的静态属性：

// 约束类包含对应静态属性，类型与方法参数匹配
const Class = this.constructor as { [K in typeof param.dataProvider]: Args };
const newArgs = Class[param.dataProvider];

内容的提问来源于stack exchange，提问作者Mike Lischke